1. Field of the Invention
This invention relates to a new and improved electrical heating element composition. More especially this invention relates to an electrical heating element composition based upon fused magnesium oxide containing as an additive therefor a composition with has excellent lubricating properties in respect of crystals of the fused magnesium oxide and which reacts at 800.degree. to 1050.degree. C with active conductivity centers at the surface of one or more adjacent magnesium oxide crystals. More especially this invention is directed to the preparation of improved electrical insulating materials employed in the tubular heating elements wherein there is present in the electrical insulating material, as an additive, a minor percentage of an additive composition which reacts with the conductivity centers of a fused magnesium oxide base whereby to decrease the electrical conductivity of the heating element and concomitantly improve its resistance characteristics. More especially this invention is directed to an electrical heating element composition containing as an additive a mineralogical composition within the MgO--SiO.sub.2 --Al.sub.2 O.sub.3 system, especially for a composition containing a major amount of components which are amorphous or radioamorphous.
2. Discussion of the Prior Art
Fused magnesium oxide is used as an electrical insulating material in tubular heating elements between the voltage-carrying heating coil and the tubular jacket. Tubular heating elements of this kind are used in the electrical heating and household appliance industries. The fused magnesium oxide has approximately the following chemical composition:
MgO: 94 - 98 weight percent PA0 SiO.sub.2 : 1.0 - 3.5 weight percent PA0 CaO: 0.5 - 2.0 weight percent PA0 Al.sub.2 O.sub.3 : 0.02 - 0.25 weight percent PA0 Fe.sub.2 O.sub.3 : 0.01 - 0.1 weight percent PA0 NiO: 0.01 - 0.03 weight percent
In addition, traces of So.sub.3, Cl, B.sub.2 O.sub.3, TiO.sub.2, Na.sub.2 O or K.sub.2 O are present.
The grain size composition of the commercial mixtures of fused and granulated magnesium oxide ranges as a rule between 0.01 and 0.4 mm. The electrical resistance of the insulating composition prepared therefrom differs greatly. Particularly when the insulating material is exposed to temperatures of over 800.degree. C, fluctuations of the electrical resistance take place. The cause of this lies in the different concentrations of the conductivity centers, as they are called in the MgO insulator.
In contrast to the ideal insulator, which theoretically has the ideal crystal lattice, i.e., a lattice having no dislocations, no electron vacancies (electron "holes") and no excess electrons, every insulator that can be made in practice has a more or less high concentration of lattice disclocations, holes and excess electrons, and this is responsible for higher or lower electrical conductivity. This concentration of defects, generally known as "imperfection", is distributed in the interior and on the surface of the crystal (cf. Fritz Rohm, "Festkorperphysik" and W. Finkelnburg, "Einfuhrung in die Atomphysik").
It is known that the concentration of imperfections can be reduced by calcining processes to an equilibrium for the specific temperature, due to the thermal vibration of the atoms in the lattice that is associated with the calcining. This knowledge has long been applied in the preparation of electrically fused magnesium oxide as an insulator. This calcination process usually follows a comminuting process performed on the raw magnesium oxide heating coil composition.
In the production of tubular heating elements, the magnesium oxide filling is again subjected to severe stress by a compressing process -- hammering, rolling and/or pressing. Due to lattice tension in the crystal grain, at the surface thereof, or due to grain destruction as a result of the mechanical stress produced by the compression, disturbances are again produced in the interior and/or on the surface of the crystals, which again result in an increase in electrical conductivity.
In practice, the quality of tubular heating elements is judged on the basis of the measured leakage currents, which are inversely proportional to the electrical resistance. These leakage currents vary in different insulating materials in spite of similar or identical composition. In particular, when a specific surface dissipation of, for example, 10 Watts per square centimeter of the surface of the element is reached, leakage currents are obtained under the test conditions stated below of between 6 mA and about 40 mA. In tubular heating elements, however, the lowest possible electrical conductivity is desired, i.e., a high electrical resistance at high temperatures and high specific electrical dissipation.
German Pat. No. 1,921,789 discloses fillings for tubular heating elements, which consist of granulated, fused MgO and the addition of sintered magnesium silicates, magnesium oxide, or mixtures thereof, the grains of the additive consisting mainly or virtually entirely of a plurality of individual crystals under 10 microns. Such fillings have an improved electrical resistance in comparison with other known fillings. It is a disadvantage of these fillings based on German Pat. No. 1,921,789, however, that they have a comparatively high electrical resistance at specific disipations of 7 to less than 9 Watts per cm.sup.2, but at dissipations of 9 to 10 W/cm.sup.2, they are only partially satisfactory in practice.
Accordingly, it is an object of the present invention to provide filling compositions performing tubular electrical insulation elements which have improved electrical resistance both at high specific dissipations of 10 W/cm.sup.2 and that the specific dissipation encountered in practice was generally less than 10 W/cm.sup.2. More especially, it is an object of the present invention to provide an improved electrical heating element composition whereby there is a marked reduction in the electrical conductivity of the resultant tubular element coupled with an improvement in the electrical resistance. More especially it has become an object of the present invention to provide an electrical heating element composition which functions to improve the respective lubricity characteristics of crystals or granules of fused magnesium oxide heating element components which will, in addition thereto, react with the conductivity centers thereof to reduce the electrical conductivity of the heating element, especially a composition which will react with the fused magnesium oxide of the electrical heating element composition at low temperatures, say, at 800- 1050.degree. C to provide a composition having overall improved electrical resistance.